Rechargeable batteries that can be used repeatedly by charging, i.e., so-called secondary batteries have increasingly been developed as power sources of portable electronic equipment. These secondary batteries include a lithium ion secondary battery. The lithium ion secondary battery can be charged and discharged with the use of the movement of lithium ions between a positive electrode and a negative electrode through an electrolyte that is disposed between the positive electrode and the negative electrode. The lithium ion secondary battery is advantageous, e.g., because the energy density is high and the so-called memory effect is small. Therefore, the lithium ion secondary battery has been the mainstream of the secondary battery for a power source to be used in portable electronic equipment, electric vehicles, etc.
On the other hand, the lithium ion secondary battery cannot avoid degradation due to repeated charging and discharging. When the lithium ion secondary battery is charged and discharged at least a certain number of times, the capacity is reduced so that the battery will reach the end of its life. This makes it difficult to use the battery continuously for a long time from then. In particular, when the lithium ion secondary battery is used as a power source of, e.g., a medical assistive device such as a hearing aid, a reduction in the voltage of the battery during the continuous use can cause the operation of the device to stop suddenly, which may lead to a serious situation that poses a potential threat to the safety of the user.
Therefore, there has been a great need for an estimation of the life of the lithium ion secondary battery, i.e., for a more accurate estimation of how much longer the lithium ion secondary battery can be used safely.
In order to meet such a need, conventional methods have been proposed to detect the life or the degree of the degradation of a lithium ion secondary battery. For example, Patent Document 1 proposes a method that includes measuring a plurality of open circuit voltage values of a battery after discharging in different charge/discharge cycle numbers, and estimating the life of the battery from the voltage values. Patent Document 2 proposes a method that includes calculating a substantial capacity of a lithium ion secondary battery when it is charged from a voltage value after a predetermined time has elapsed during the constant-current charging, and comparing the capacity with the initial capacity to estimate the degree of the degradation of the battery.